1. Field of the Invention
This invention relates to a silver halide photographic emulsion, and more particularly to a silver halide photographic emulsion which is spectrally sensitized with a combination of a red- or green-sensitizing dye with a blue-sensitizing dye in which the combination of both dyes exhibits a supersensitizing effect on each other.
2. Description of the Prior Art
IN SILVER HALIDE PHOTOGRAPHIC LIGHT-SENSITIVE MATERIALS HAVING PANCHROMATIC SPECTRAL SENSITIVITY CHARACTERISTICS, A THIACARBOCYANINE DYE OR A SELENACARBOCYANINE DYE IS FREQUENTLY USED AS SENSITIZING DYE FOR THE SPECTRAL SENSITIZATION OF A SILVER HALIDE PHOTOGRAPHIC EMULSION. However, panchromatic photographic light-sensitive materials sometimes require further spectral sensitization to blue light ranging from wavelengths shorter than 500 nm to wavelengths longer than the inherent light-sensitive wavelength region of the silver halide. For example, this is required for panchromatic photographic light-sensitive materials used for three-color separation photographing in the plate making process for printing. For this purpose, it is known to use an apomerocyanine and/or a dimethinemerocyanine dye together with the above thiacarbocyanine or selenacarbocyanine dye (for example, as disclosed in U.S. Pat. No. 3,808,009). However, the combined use of such a merocyanine dye with a thiacarbocyanine or selenacarbocyanine dye often results in a reduction in sensitivity to red light. In particular, when the above merocyanine dye and carbocyanine dye are stored as a mixed solution and then used in the production of light-sensitive materials, there are tendencies toward a reduction in the sensitivity of the photographic emulsion and toward the occurrence of fog. In the practical production process, the addition of two or more different dyes as a mixed solution thereof can be used to reduce the amount of a solvent used for dissolving the dyes, and simplifies the facilities required and process control needed as compared with the addition thereof as separate solutions. Therefore, discovery of a combination of sensitizing dyes which are stable also in the form of a mixed solution thereof has been desired.
On the other hand, the spectral sensitivity characteristics, particularly the spectral sensitization maximum wavelength of a photographic emulsion layer is important in photographic light-sensitive materials. In color negative photographic light-sensitive materials used for photography or color reversal photographic light-sensitive materials, the sensitization maximum wavelength of the red-sensitive layer is adjusted to an appropriate value in order to obtain the desired color reproducibility, considering the relation thereof with the spectral characteristics of the blue-sensitive layer and the green-sensitive layer. For example, a styryl dye which itself scarcely provides any spectral sensitization is used together with a carbocyanine dye having a red-sensitizing effect. For example, the combinations as described in U.S. Pat. Nos. 3,615,635 and 2,533,426 can be used, by which the sensitization maximum is shifted to a shorter wavelength than that obtained with the use of the carbocyanine dye alone. However, styryl dyes have a relatively low supersensitizing effect when combined with a carboxyanine dye. If the styryl dye is added in an amount sufficient to shift the sensitization maximum to a shorter wavelength, the red-sensitivity is reduced in most cases. In addition, many styryl dyes are unstable, so that care must be taken in handling solutions of these dyes.